Political activists opposed to US President George W Bush have been told they will not be allowed to stage a huge rally in New York this weekend.

The planned anti-war protest in Central Park was to coincide with the eve of the Republican Convention and would have begun a week of protest rallies.

It meant to make life uncomfortable for the president and his supporters.

Organisers say the ban is a violation of their constitutional rights of assembly and free speech.

A judge at New York's state Supreme Court has ruled that the rally, which was expected to attract a quarter of a million demonstrators, cannot be held in Central Park because of the damage which may be caused to the grass.

The city has given permission for a march to pass the convention centre in midtown Manhattan on Sunday.

However, organisers have warned that in the absence of a place to gather legally, demonstrators may move on Central Park afterwards and that could bring the possibility of provoking clashes with the police.

Leslie Cagan, co-ordinator for the United for Peace and Justice group, said the ban on the demonstration was politically motivated by the Republican Mayor of New York, Michael Bloomberg.

"A Republican mayor hosting a Republican convention has done everything designed to undermine the demonstration against policies of a Republican administration," she said.